Index: wp-admin/includes/plugin.php
===================================================================
--- wp-admin/includes/plugin.php	(revision 37753)
+++ wp-admin/includes/plugin.php	(working copy)
@@ -1057,7 +1057,7 @@
  * @param string   $page_title The text to be displayed in the title tags of the page when the menu is selected.
  * @param string   $menu_title The text to be used for the menu.
  * @param string   $capability The capability required for this menu to be displayed to the user.
- * @param string   $menu_slug  The slug name to refer to this menu by (should be unique for this menu).
+ * @param string   $menu_slug  The slug name to refer to this menu by (should be unique for this menu and only include lowercase alphanumeric, dashes and underscores characters to be compatible with the sanitize_key() function).
  * @param callable $function   The function to be called to output the content for this page.
  * @param string   $icon_url   The URL to the icon to be used for this menu.
  *                             * Pass a base64-encoded SVG using a data URI, which will be colored to match
@@ -1127,7 +1127,7 @@
  * @param string   $page_title  The text to be displayed in the title tags of the page when the menu is selected.
  * @param string   $menu_title  The text to be used for the menu.
  * @param string   $capability  The capability required for this menu to be displayed to the user.
- * @param string   $menu_slug   The slug name to refer to this menu by (should be unique for this menu).
+ * @param string   $menu_slug   The slug name to refer to this menu by (should be unique for this menu and only include lowercase alphanumeric, dashes and underscores characters to be compatible with the sanitize_key() function).
  * @param callable $function    The function to be called to output the content for this page.
  * @return false|string The resulting page's hook_suffix, or false if the user does not have the capability required.
  */
Index: wp-admin/includes/template.php
===================================================================
--- wp-admin/includes/template.php	(revision 37753)
+++ wp-admin/includes/template.php	(working copy)
@@ -887,7 +887,10 @@
  * @param string|array|WP_Screen $screen        Optional. The screen or screens on which to show the box
  *                                              (such as a post type, 'link', or 'comment'). Accepts a single
  *                                              screen ID, WP_Screen object, or array of screen IDs. Default
- *                                              is the current screen.
+ *                                              is the current screen.  If you have used add_menu_page() or 
+ *                                              add_submenu_page() to create a new screen (and hence screen_id)
+ *                                              make sure your menu slug conforms to the limits of sanitize_key()
+ *                                              otherwise the 'screen' menu will not appear on your page.
  * @param string                 $context       Optional. The context within the screen where the boxes
  *                                              should display. Available contexts vary from screen to
  *                                              screen. Post edit screen contexts include 'normal', 'side',
@@ -982,7 +985,10 @@
  * @global array $wp_meta_boxes
  *
  * @staticvar bool $already_sorted
- * @param string|WP_Screen $screen  Screen identifier
+ * @param string|WP_Screen $screen  Screen identifier.  If you have used add_menu_page() or 
+ *                                  add_submenu_page() to create a new screen (and hence screen_id)
+ *                                  make sure your menu slug conforms to the limits of sanitize_key()
+ *                                  otherwise the 'screen' menu will not appear on your page.
  * @param string           $context box context
  * @param mixed            $object  gets passed to the box callback function as first parameter
  * @return int number of meta_boxes